Throughout his presidential campaign, Trump often characterized these locales as dangerous hotbeds of criminal activity and promised to suspend all federal funding to them.

“We will end the sanctuary cities that have resulted in so many needless deaths,” he told a crowd in Phoenix in the fall.

But an analysis of FBI crime data by Tom Wong, a professor of political science at the University of California at San Diego, finds that counties designated as “sanctuary” areas by ICE typically experience significantly lower rates of all types of crime, including lower homicide rates, than comparable non-sanctuary counties. The analysis was published by the Center for American Progress, a progressive think tank.

President-elect Donald Trump pledged to end "sanctuary cities" while campaigning for the White House. Washington, D.C., is one such city. Here's what that means and how D.C. is trying to fight Trump's stance. (Claritza Jimenez/The Washington Post)

In 2015, the typical sanctuary county in a large metropolitan area experienced 654 fewer crimes per 100,000 residents than the typical non-sanctuary county in a big, metropolitan area. That's an overall crime rate approximately 15 percent lower.

In smaller counties and even rural areas, crime rates were also lower for sanctuary areas, Wong found.

The exception is medium metros and counties on the fringes of large metro areas, which had slightly higher crime rates if they were sanctuary areas.

Overall, across all counties, there are on average 355 crimes per 100,000 in sanctuary counties.

Specifically addressing Trump's contentions that sanctuary cities are magnets for homicides, Wong found that the typical sanctuary area saw 1 fewer homicide per 100,000 people in 2015 than the typical non-sanctuary area. While the difference is small, Wong's statistical tests indicate it is highly significant.

“The data are clear that sanctuary counties aren't crime-ridden hellholes,” Wong said in an interview.

The data only shows correlation; Wong says more research needs to be done to determine whether a causal effect is at work here. But he said he suspects that, by becoming a sanctuary area and refusing to involve local authorities in deportation matters, a city or county may actually make itself safer. If immigrants who came to the United States illegally fear working with police will lead to deportation, they're less likely to report crimes and assist with investigations.

This is the position of a number of law enforcement groups. The Major Cities Chiefs Association, representing the 63 largest urban areas in the United States, said that “immigration enforcement by local police would likely negatively effect and undermine the level of trust and cooperation between local police and immigrant communities,” which would “would result in increased crime against immigrants and in the broader community, create a class of silent victims and eliminate the potential for assistance from immigrants in solving crimes or preventing future terroristic acts.”

Similarly, the International Association of Chiefs of Police said that “state and local law enforcement should not be involved in the enforcement of civil immigration laws since such involvement would likely have a chilling effect on both legal and illegal aliens reporting criminal activity or assisting police in criminal investigations.”

Wong also looked at a number of economic indicators in his analysis and found that in sanctuary counties, income tends to be higher while poverty tends to be lower. He said the he suspects a causal mechanism at work here, too.

“If you deport the breadwinner [of an immigrant family], that leaves families more economically vulnerable,” he said. “That means that these economically vulnerable families are more reliant on public assistance.”